Traditionally, a subsea christmas tree provides pressure control of a well completion system that comprises a centrally located well bore and a surrounding annulus conduit. The centrally located well bore is typically used for the extraction of reservoir hydrocarbons and is referred to as the production bore. The annulus conduit is typically used to service the well, for example allowing the circulation of fluids during well start up and shut down. During the production phase of the well, the annulus is often redundant and is monitored for pressure build up indicating a possible production tubing or packer leak from the production bore. Some wells employ the annulus for gas lift. Gas is pumped down the annulus and enters the production bore at specific locations thereby reducing the density and viscosity of the produced fluids. Electrical, optical and hydraulic service lines are also typically routed through the annulus for powering and control of downhole equipment such as valves and pumps, or for data transmission from downhole sensors. Chemical injection lines are likewise routed through the annulus.
Recent developments in expandable casing technology and reeled tubular technology dictate completion designs having decreased diameter well casing tubulars located external to the production tubing. The radial gaps between the tubulars are likewise reduced.